1. Field of the Invention
The present invention generally relates to a magnetic resonance imaging method and a magnetic resonance imaging system for producing a spectroscopic image and an MR (magnetic resonance) image of a biological body under medical examination, such as a brain of a patient. More specifically, the present invention is directed to a specific spin echo method and a system using this specific spin echo method capable of acquiring short "T.sub.2 " signal components contained in an MR signal.
2. Description of the Prior Art
In the conventional MR spectroscopic imaging (referred to an "MRSI"), there are provided typical two pulse sequences, namely the FID (free induction decay) method with utilizing the selective excitation, and the spin echo method. In accordance with the spectroscopic imaging by employing the known FID pulse sequence, one of the gradient fields along the orthogonal 3-axis directions is used as the slicing gradient field, whereas the remaining two fields are used as the phase-encoding gradient fields. Under such a gradient field relationship, the slicing gradient field is applied to a biological body under medical examination with an application of a selective 90.degree. pulse, and then is ramped down and finally reaches a zero level. Furthermore, when the phase-encoding gradient fields are applied as same as when the slicing gradient field for the rephasing purpose is applied, and the application of the phase-encoding gradient fields is accomplished, the FID signal is acquired. In this FID method, a time period from a center of the selective excitation 90.degree. pulse until the acquisition of the FID signal, namely the delay time is prolonged, during which the above-described applications of the various gradient fields are carried out. Accordingly, the resultant spectrum contains a baseline distortion. Furthermore, there is another problem in this FID method that the longer the delay time becomes, the more the FID signals of the short T.sub.2 -relaxation time are lost, or attenuated.
Also, in case of the spectroscopic imaging with use of the pulse sequence by the spin echo method, the following pulse sequence operation is carried out. That is, after a selective excitation 90.degree. pulse has been applied to a biological body under medical examination and a half of echo time "T.sub.E " (namely, 1/2 T.sub.E =.tau.) has elapsed, a non-selective 180.degree. pulse is applied thereto, and thereafter spin echo signals from the biological body are acquired after another half of echo time "T.sub.E " has elapsed. Although the relationship among the orthogonal 3-axis gradient fields of the spectroscopic imaging by this spin echo method is similar to that of the FID method, the spin echo signals are acquired just after 1/2 T.sub.E has elapsed since the non-selective excitation 180.degree. pulse had been applied. In other words, the MR signal (spin echo signal) acquisition of the spin echo method starts earlier than the MR signal (FID signal) acquisition of the FID method. Moreover, in case of this spin echo method, since the dephasing phenomenon of the spin which happens to occur after the selective excitation 90.degree. pulse due to the disturbance in uniformity of magnetic fields, can be rephased by the non-selective 180.degree. pulse to acquire the spin echo signals, there is another merit that no baseline distortion is contained in the spin echo signal.
However, when the spectroscopic imaging is performed by utilizing the pulse sequence of the conventional spin echo method, since the selective excitation slicing gradient field is first applied to the biological body, and thereafter the dephased spins caused by this slicing gradient field are compensated by applying the non-selective excitation 180.degree. pulse, the overall echo time "T.sub.E " cannot be considerably shortened. Even if the application time duration of the selective excitation slicing gradient field would be shortened as much as possible, the overall echo time "T.sub.E " could be more or less shortened. As a consequence, since the total echo time "T.sub.E " is still long, the spin echo signals having short "T.sub.2 " (spin-to-spin relaxation) time would be lost, namely low S/N ratios. For example, generally speaking, T.sub.2 of water is approximately 100 milliseconds, whereas T.sub.2 of .gamma.-ATP is about 10 to 15 microseconds, which could not be acquired at the practically acceptable signal level, or satisfied S/N ratio in the conventional spectroscopic imaging with employment of the pulse sequence of the conventional spin echo method.